The liquid crystal display is mainly composed of polarization plates, an array substrate, a color filter substrate, a liquid crystal layer and the like. The array substrate and the color filter substrate are bonded together to form a liquid crystal cell structure, in which the liquid crystal layer is filled. The polarization plates are adhered to the back surface of the array substrate and the color filter substrate, respectively. The thickness of the liquid crystal layer, also referred as a cell thickness, may have a great influence on the light adjustment of the liquid crystal. Consequently, during the manufacturing of the liquid crystal display, it is important to ensure the uniformity of the cell thickness for realizing a high quality liquid crystal display.
In order to ensure the uniformity of the cell thickness, a method of providing a spacer is typically employed in the prior art. Currently, the spacer can be classified into two categories of ball spacer (BS) and photo spacer depending on its shape.
The ball spacers are typically distributed on the array substrate or the color filter substrate by a spraying method. After the panel is completed, the relative uniform cell thickness is realized depending on the uniformity of the size of the ball spacers. However, since the distributing process of the ball spacers is highly random, it is difficult to control the uniformity of the density; in turn it is difficult to achieve a desired level for the uniformity of the cell thickness, and the adverse effects such as the light scattering will occur.
In order to overcome the shortcomings of the ball spacers, a photo spacer process is employed in the prior art instead. The photo spacers are formed by a lithography process on the color filter substrate during manufacturing the color filter substrate and the positions of the photo spacers on the color filter substrate can be accurately controlled, so as to increase the uniformity of the cell thickness and improve contrast.
In the current design of the photo spacers, a structure of combination of primary photo spacers and secondary photo spacers is typically employed. The primary photo spacers are used as the main part for maintaining the uniformity of the cell thickness, and are positioned at higher elevation on the surface of the array substrate, e.g., on the thin film transistors (TFT) of the array substrate. During manufacturing the panel, the primary spacers are subjected to a large deformation, and play an important role in maintaining the uniformity of the cell thickness. In contrast, the secondary photo spacers are typically positioned at lower elevation on the surface of the array substrate, e.g., on the gate lines. During manufacturing the panel, the secondary photo spacers are subjected to a small deformation, if not at all, therefore, the secondary photo spacers have a minor effect on maintaining the uniformity of the cell thickness. The secondary photo spacers are mainly used to prevent excessive stress applied on the primary photo spacers due to the small supporting area of the primary photo spacers when the liquid crystal display is under a relatively large external force, so as to avoid occurrence of the unrecoverable change of the cell thickness or damage of the structure the photo spacers.
FIG. 1 is a schematic view of the channel location in the array substrate in the prior art. The reference “A” shows the position of the channel. FIG. 2 is a schematic view showing the primary and secondary photo spacers in the photo spacer design in the prior art. The reference “1” designates the primary photo spacer and the reference “2” designates the secondary photo spacer. As for the design for the photo spacer in the prior art, a sub-pixel is typically adopted as a reference, and the primary photo spacers are positioned at a specific location relative to the sub-pixel. Since the TFT has a higher elevation with a relatively large area in the sub-pixel, and becomes one of the major location for placing the primary photo spacers. However, the surface of the TFT is not flat due the presence of the channels. Although the specific location of the photo spacer in the sub-pixel is selected, the alignment accuracies for the different sub-pixels are different, so that the actual contact area between the primary photo spacers and the TFT will vary when the contact position between them are different. Therefore, the deviation of the alignment accuracy between the color filter substrate and the array substrate in the panel will lead to a difference in the support force by the primary photo spacers.
Under the current manufacturing condition for the liquid display panel, the assemble accuracy between the color filter substrate and the array substrate is typically in an order of several microns, i.e., an inevitable deviation of several microns may present for relative position between the array substrate and the color filter substrate assembled together. Therefore, after the array substrate and the color filter substrate are assembled, the actual position of the primary photo spacers relative to the array substrate will also deviate from the predetermined specific location unavoidably. Under the condition of the alignment deviation for several microns, the area in which the primary photo spacers overlap with the TFT channel will vary, so that the variation in the actual deformation amounts for the primary photo spacers will occur and the uniformity of the cell thickness will be decreased.
In addition, the width of the TFT channel may be 3-6 μm, and the size of the photo spacer may be about 6-17 μm. In the case of one drop filling (ODF) process is employed, in order to ensure the allowable deviation of the liquid crystal drop amount, the support area of the primary photo spacer is typically required to be as small as possible, and the size of the primary photo spacer is required to be as small as possible. However, due to the presence of the above problems, when the size of the primary photo spacer is reduced, the influence on variation of the support force by the primary photo spacer with the alignment deviation will become greater. FIGS. 3, 4 and 5 shows distribution views of the support area on the TFT under the following conditions in the prior art: the diameters of the primary photo spacers are 12 μm, 10 μm and 9 μm, all of the primary photo spacers are at the same prescribed location, and the alignment accuracy of the primary photo spacers is 5 μm. In FIGS. 3, 4 and 5, the horizontal ordinate and the vertical ordinate show the ranges of alignment accuracy for the array substrate and the color filter substrate (unit: μm), respectively, with the different colors showing the actual support areas for the different primary photo spacers (an average value of 1 is defined for the support area of the primary photo spacer 1 within an alignment accuracy of 5 μm).
On one hand, for the ODF process, the design of the primary photo spacer is directed to have smaller size and lower density, so as to ensure a large allowable deviation of the liquid crystal drop amount and increase the yield of the panel; on the other hand, with the limitation of the current process, when the primary photo spacer is provided on the TFT, although all of the primary photo spacers are designed to have the same prescribed specific location within the sub-pixels, the actual positions of the primary photo spacers within the sub-pixel will be different due to the different alignment deviations in the different positions inside the panel. Therefore, the actual support force by the primary photo spacers at the different positions inside the panel will be different with the different alignment deviations due to the presence of the TFT channel, and the smaller the size of the primary photo spacer is, the larger difference of the actual support force will be. Consequently, quality problems such as touch mura will be more likely to occur.